Patent Document 1, for example, is known as a conventional technique for supplying back-up power to a shift control unit included in a shift-by-wire device provided in a vehicle, as mentioned above. Patent Document 1 discloses a technique for supplying back-up power from a capacitor (a back-up power source) to an SBW-ECU (shift control unit) when a power source such as a battery, a wire in a power supply system, or the like provided in a vehicle has malfunctioned, i.e., in the event of a battery failure causing the supply of power from the battery to be lost. According to the technique disclosed in this document, a control signal from the SBW-ECU supplied with power switches the shift range of an automatic transmission to the parking range (P range), which makes it possible to safely park the vehicle.
However, as mentioned in the aforementioned document as well, a user of a vehicle may wish to specify the timing at which the shift range is switched to the parking range (P range) in order to park the vehicle in a predetermined location in the event of a battery failure. In this case, according to the technique of the aforementioned document, it is necessary to supply power from the back-up power source to the SBW-ECU from when the battery failure has occurred to when the parking range (P range) is switched to. There is thus a problem in that the capacity of the back-up power source must be high enough to provide the required supply. A higher capacity increases the size of the back-up power source.
For design purposes and the like, it is also conceivable that the back-up power source device which controls the supply of power from the back-up power source, such as a capacitor, to the SBW-ECU (the shift control unit), is provided separate from the shift control unit. Thus what is needed is a technique in which a back-up power source can be used efficiently in a situation where, in the event of a battery failure, the shift range is to be switched to the parking range in response to a user instruction, and where the back-up power source device and the shift control unit are provided separately.
Having been achieved on the basis of circumstances such as those described above, the technique disclosed in the present specification provides a back-up power source device which, in the event of a battery failure, can efficiently use a back-up power source for a shift control unit when switching a shift range to the parking range in response to a user instruction, and which can be made smaller.